The present invention relates to a method and system for determining the viscosity and the viscosity index of a product. More particularly, the present invention relates to a method and system for sampling a stream, e.g., petroleum product stream, and for automatically determining the viscosity at any temperature and the viscosity index (VI) of the petroleum product.
Dynamic viscosity can be determined by measuring the force required to overcome fluid friction in a film of known dimensions. Dynamic viscosity is usually reported in centipoise or Pascal seconds (Pas) (1 cP=10.sup.-3 Pas) and is determined using well known equations such as Poiseuille equation: ##EQU1## wherein .mu.=absolute (dynamic) cP
d=inner dia of conduits, mm, PA1 L=length of segments, mm, PA1 Q=flow rate in conduits, ml/min, and PA1 .DELTA.p=press drop across segments, bars. PA1 where PA1 L is the kinematic viscosity at in cS (mm.sup.2 sec.sup.-1) at 40.degree. C. of an oil of 0 viscosity index having the same kinematic viscosity at 100.degree. C. as the oil whose viscosity index is to be calculated, PA1 H is the kinematic viscosity at in cS (mm.sup.2 sec.sup.-1) at 40.degree. C. of an oil of 100 viscosity index having the same kinematic viscosity at 100.degree. C. as the oil whose viscosity index is to be calculated, PA1 U is the kinematic viscosity at 40.degree. C. of the sample oil. PA1 .DELTA.p.sub.2 =pressure drop-second conduit (bars), PA1 T.sub.1 =absolute temperature-first slip-stream portion, PA1 T.sub.2 =absolute temperature-second slip-stream portion. PA1 z=(V+0.7+C-D+E-F+G-H); PA1 log=logarithm to base 10; PA1 v=kinematic viscosity,cSt (or mm.sup.2 /s); PA1 T=temperature, K or .degree.R; PA1 A&B=constants; and
L and d are fixed, Q may be fixed or measured, and dp is measured. For a given L, d and Q, Viscosity is a constant times the .DELTA.p.
Kinematic viscosity is a measure or timing of the flow driven by gravity, and is the viscosity most often use in a laboratory. Dynamic viscosities, in centipoise, can be converted to kinematic viscosities, in centistokes (cSt), by dividing by the density in grams per cubic centimeter (g/cm.sup.3) at the same temperature. The kinematic viscosity unit is mm.sup.2 /sec or centistokes.
The viscosity of a fluid usually increases as temperature is decreased, and decreases as the temperature is increased. Viscosities of oils at temperatures other than the temperature at which they are measured can be determined by measuring the viscosities at two temperatures, then plotting these points on viscosity-temperature charts developed by ASTM and issued as standard ASTM D341 which is incorporated herein by reference. A straight line is drawn through these two points, and viscosities at other temperatures are read along this line.
Different oils have different rates of change of viscosity with temperature. Viscosity Index (VI) is a method of representing this rate of change, based on a comparison with the relative rates of change of two arbitrarily selected types of oils that differ widely in this characteristic. A high VI indicates a relatively low rate of change of viscosity with temperature. Conversely, a low VI indicates a relatively high rate of change of viscosity with temperature.
VI is commonly associated with petroleum products such as crude oils, lubricating oils and their constituent base stocks. It is one of the most important specifications that must be met when determining the suitability of a particular oil for a given use. This characteristic is important because many oils are expected to operate not at some fixed temperature but over a range of temperatures. For example in the case of an automobile engine, a lubricating oil is expected to perform properly over a potentially very wide range of ambient and operating temperatures. These temperatures can vary from below freezing to the very high temperatures observed in tropical zones.
An ASTM standard provides a widely used scale for reporting VI. The scale is an empirical one based on the arbitrary assignment of VI values to two different kinds of crude oils. The viscosity-temperature relationship of a Pennsylvania crude was arbitrarily assigned a VI of 100. That same relationship for a Gulf Coast crude was assigned a value of 0. Some lubricating oils particularly synthetic oils may have a VI below 0 or above 100. The use of additives can increase observed VI to well over 100.
This method for measuring the VI of an oil is the standard described in ASTM D 2270-91 which is incorporated herein by reference. This standard describes a method for determining the VI of petroleum products using their kinematic viscosities at 40.degree. C. and 100.degree. C. For oils of Viscosity Index up to and including 100, VI may be calculated using the equation: EQU VI=(L-U)/(L-H)!.times.100
For oils having a VI over 100, equation I does not give reliable results and an alternate procedure detailed in ASTM D 2270 must be followed. The use of the ASTM method requires maintaining constant sample temperatures at the 40.degree. C. and 100.degree. C. points while viscosity measurements are made. Thus, the ASTM procedure can require expensive constant temperature bath equipment and a great deal of time to produce results. Much of that time required is devoted to taking a sample from constant initial temperature of 40.degree. C. to a constant 100.degree. C. For the process operator desiring rapid VI information that could be of use to monitor product quality during manufacture, this delay is unacceptable.
Devices are known for measuring viscosity. However, none provide direct information concerning the VI of the liquid or other substance being tested. In view of the importance of VI to users of petroleum products, particularly lubricating oils, there is a need for a device that provides direct, continuous information concerning VI.